This invention relates to the application of micromechanical machining to cooling of integrated circuits.
Certain integrated circuits operate at subambient temperatures. For example, cryoelectronic integrated circuits, such as Josephson junction (JJ) circuits, operate at cryogenic temperatures, i.e. temperatures below about 100K.
It is desirable to avoid thermally stressing an integrated circuit that is designed to operate at subambient temperatures, and this implies that the circuit structure should be maintained at subambient temperature throughout its working life. A typical integrated circuit is expected to operate for at least two years, and consequently a cooling device that is used to maintain a cryoelectronic integrated circuit at a cryogenic temperature must remain in service for at least two years without failing. The design and manufacture of a cooling device that will operate for such an extended period of time is attended by severe practical difficulties. For example, many conventional cooling devices operate by successively compressing and expanding a gaseous refrigerant. Typically, the refrigerant is compressed by use of a motor-driven pump and is then allowed to expand through an orifice. Such cooling devices have parts that move relative to each other in sliding contact, and these sliding parts are not well suited to continuous use for long periods of time.
A further requirement with respect to cooling of cryoelectronic integrated circuits relates to the size of the cooling device. It is desirable that the cooling device and the cryoelectronic integrated circuit be integrated as a single unitary structure, and that this structure not be significantly more bulky than the package of a conventional multichip module, which may be, for example, 8 cm.sup.3 in volume.
G. W. Swift, Thermoacoustic Engines, J. Acoust. Soc. Am., Vol. 84 (No. 4) pg. 1145 (October 1988), the disclosure of which is hereby incorporated by reference herein, describes a thermoacoustic refrigerator having no moving parts. The thermoacoustic refrigerator comprises a prime mover (or thermoacoustic driver) that produces acoustic work in response to a temperature gradient and a heat pump that is driven by the acoustic work produced by the prime mover. The two thermoacoustic engines (the prime mover and the heat pump), which operate based on similar principles, each comprise a wall that bounds a cylindrical cavity for supporting an acoustic standing wave, and two heat exchangers intermediate the ends of the cavity. The prime mover has a hot heat exchanger, which receives heat from an external source, and a room temperature heat exchanger, whereas the heat pump has a cold heat exchanger, which absorbs heat, and a room temperature heat exchanger. When heat is applied to the hot heat exchanger of the prime mover, heat is absorbed by the cold heat exchanger of the heat pump and is rejected by the room temperature heat exchangers.
Each engine employs two thermodynamic media, one of which is a working fluid inside the cavity. The second thermodynamic medium is a stack of plates between the heat exchangers, the plates being spaced apart by a distance related to the thermal penetration depth in the fluid. The plates are made of a material having poor thermal conductivity, typically stainless steel, so that when one heat exchanger is hot and the other heat exchanger is cold, a temperature gradient exists along the plates. The room temperature heat exchangers are typically made of copper plates, the copper plates being 0.4 mm thick and at a spacing of 2.0 mm between plates for an operating frequency of 400 Hz. The hot heat exchanger of the prime mover may be made of nickel plates, and the cold heat exchanger of the heat pump may be made of copper plates.
The thermoacoustic refrigerator described by Swift is not suitable for cooling a cryoelectronic integrated circuit, because it is much too large to incorporate in a standard package for an integrated circuit. Further, for a thermoacoustic refrigerator that is sufficiently small to incorporate in an integrated circuit package, for example about 5 mm long, the stainless steel plates that form the second thermodynamic medium would need to be at a spacing of about 0.04 mm, and it has not been demonstrated that a suitable stack of stainless steel plates can be manufactured with any degree of consistency. Moreover, fabrication of suitable heat exchangers for a very small thermoacoustic refrigerator presents substantial difficulties.
The entropy of a body of material is lower when the body is cold than when it is hot. Thus, if a body of magnetic material in which the spins are randomly oriented is placed in a magnetic field such that the spins become aligned, its entropy is reduced and so also is its temperature. If the body of magnetic material is then placed in thermally conductive contact with an integrated circuit, heat flows from the integrated circuit to the body of magnetic material. Therefore, if the thermally conductive contact between the circuit and the body of magnetic material is accomplished by use of a thermal switch, i.e. a switch that selectively conducts heat, and the switch is operated synchronously with change in the magnetic field, the body of magnetic material can be used to cool the integrated circuit. The body of magnetic material then serves as a solid state cryogen.
A known superconductive material has a low thermal conductivityi when it is below the transition temperature and is in the superconductive state and a high thermal conductivity when it is below the transition temperature and is in the normally conductive (i.e. non-superconductive) state, and can be switched between the superconductive state and the normally conductive state by change in magnetic field. Thus, a wire of superconductive material can be used as a thermal switch that has no moving parts. However, this effect is of limited utility because separate means must be employed for cooling the wire to below the transition temperature.
Micromechanical machining is a technique whereby mechanical parts are formed using processes, such as photolithography and etching, that were developed and refined for use in manufacture of integrated circuits. Micromechanical machining allows fine control of dimensions and is commonly employed for production of mechanical parts from silicon. Micromechanical machining is not restricted in its application to forming of workpieces of silicon or other materials that are conventionally used in manufacture of integrated circuits, and it is known to apply micromechanical machining to other materials. Moreover, micromechanical machining can be used to form a mold in a body of silicon, for example, and another material that is not a suitable subject for photolithography or etching can be fabricated by reverse casting using the mold.
A typical cryogenic regenerator for an orifice pulse tube refrigerator (OPTR) operating at 400 Hz employs spheres of metal as a thermodynamic medium. The dimensions of the spheres depend upon the operating temperature, operating frequency, and the material of the spheres.